Radioactive thallium is used in an important method for diagnosing coronary artery and ischemic heart disease, and related cardiac disorders. Radioactive imaging with 201T1 is the most frequently employed method for myocardial imaging, to aid in visualizing the blood flow to the various regions of the left ventricular myocardium. High quality 201T1 is produced using the enriched stable isotope 203T1, which is increasingly less available. Recent developments in high resolution laser photoionization using new solid state continuous wave laser technology potentially permit the highly selective enrichment of the minor isotope 203T1 in natural thallium. The successful development of this technology would assure a stable and reasonably priced supply of 203T1 to support 201T1 radioactive imaging needs. The specific aims of this project are to develop a modified thermal atomization thallium source, investigate the photoionization spectroscopy of 203T1, and to determine the potential for isotopically selective ionization and enrichment of 203T1.